Mr Bryson on Baily’s Compensation Pendulum. 221. 
George Graham, the inventor of the clock escapement 
which bears his name, was the first who succeeded in com- 
pensating a pendulum for thermal changes. In December 
1721, he attached to a clock a compound pendulum composed 
of a single steel-rod, fixed to a stirrup or frame, carrying a 
glass jar filled with mercury. The effect from expansion of 
the rod downwards was thus counteracted by the increased 
length of the column of mercury raising the centre of oscilla- 
tion ; thus maintaining the equality of rate in all tempera- 
tures. 
This elegant invention was no less remarkable in its sim- 
plicity than in the fact, that not the slightest improvement 
has as yet been made on this, the first, regulator of Graham. 
The expense of this most accurate pendulum is, however, 
considerable, and many attempts have been made to obtain 
a cheaper substitute ; this desideratum has been ably sup- 
plied by the late lamented Francis Baily, Secretary of the 
Astronomical Society of London. 
This pendulum consists of a leaden cylinder attached to arod 
of the common pine (Pinus strobus), and is thus described by 
the inventor :—“ Take a cylindrical deal-rod of a convenient 
size, but not less than 46 inches in length, about 3 of an inch 
diameter ; procure a leaden cylinder, with a hole through the 
centre which will freely admit the end of the rod, and of such 
a length, that when put in the lathe it may be reduced to 
the required standard of 14.3 inches, and to the required 
weight. It will be more convenient to have this cylinder too 
long rather than too short, since we may readily diminish its 
length, if, on trial, it should be found to over-compensate the 
pendulum.’’* 
The following are the respective weights, as given by 
Baily, of a leaden cylinder 14.3 inches long, and having a 
hole through the centre equal to 3 of an inch diameter. 
Diameter of Cylinder. Weight of Cylinder. 
1} inches. 6.56 lbs. 
1 ee 9.73 ... 
ee 47: 
Zi ieee 17.80)... 
i alae’ 22.70 ... 
* These dimensions are on the assumption, that deal expands 
1000022685 of its bulk for 1° Fahrenheit, and lead .0000159200. 
